How to get a path from a circle?

[Veruschka]

Hello,

I'm just working on a model from a plane and become now desperate of trying to get a path out a circle object.
As you can see in the picture I need a path that is about the red drawn line.


Thought a circle would be a good point to start. Just had to adjust the vertices a bit and then I'm done. But when I tried to erase one vertex, hopping I would receive an open path then AC3D just draw a line between the left vertex so that it remains a closed line. Tried all I could find to get it broken (knife, extrude and erase the surfaces, creating objects from the vertices only, unwelding a vertex, line and polyline…) but with no success.
As you can see in the second picture it locked that I got what I was looking for with object 2. But as you see AC3D won't accept that path to extrude a object along it (thats what I need the path for). Is there something special about a circle object so that you couldnt convert it?



Hope anyone could help.

Regards & Thanks

Frank